Checks: Fixed scheduling of service checks

Commit 62af86766d24e73c33edb76ae01ea1ac8dcb6f04 introduced a random delay
when rescheduling all service checks. This should have only been applied
when the next check time is more than the check interval (or retry interval)
in the future because of check timeperiod constraints. This commit fixes
that issue. The logic was already correct for rescheduling host checks.